1. Field of the Invention
The present invention relates to a lock-up control system for a lock-up torque converter for a lock-up type automatic transmission.
2. Description of the Prior Art
Commonly, automatic transmissions have a torque converter in a power transmitting system not only to absorb the variation in torque generated by an engine, but also to multiply the engine torque. The ordinary torque converter operates such that an input element (turbine runner) rotates a working fluid inside the converter and the rotation of the working fluid rotates an output element (turbine runner) under the reaction of a stator by multiplying the torque (a converter state). Thus, since the input and output elements of the torque converter are not mechanically connected with each other, a slip between the input and output elements is unavoidable when it is in operation, causing the automatic transmission having the torque converter to operate with poor power transmission efficiency although it is easy to operate. Therefore, it has been proposed to provide a lock-up type automatic transmission that has a lock-up torque converter which locks up via the engagement of a lock-up clutch (lock-up state) when a vehicle speed is higher than a predetermined value (often called a lock-up vehicle speed value) in at least one gear position because in this circumstance the multiplication of the engine torque by the torque converter is not required and the variation in the engine torque is so small as to create no problem.